The invention relates to electromagnetic impact sensors, and more particularly to miniature self-contained electromagnetic impact sensors for use in security systems, gaming devices, and other applications.
Impact sensors, or shock sensors, play an important role in security systems for automobiles, buildings, and other objects, and are often used in gaming devices as well as numerous other applications to detect or count impacts. While most impact sensors have a common purpose, namely to produce an electrical signal in response to an impact or sudden acceleration, they may be embodied in any of a variety of forms. Common types of impact sensors include electromechanical and electromagnetic sensors.
One type of impact detector, an electromechanical sensor, uses an electrical contact mounted for movement with a weighted spring or pendulum. When an impact is experienced, the spring or pendulum moves, causing the first contact to touch another electrical contact. This completes an electrical circuit and triggers an alarm or other event. Electromechanical sensors have disadvantages for several reasons: they are often not easily tuned or calibrated, since their characteristics are reliant on mechanical relationships; the mechanical springs and contacts may be subject to wear and possible corrosion, changing the characteristics of the device over time; and they may not be omnidirectionally sensitive, since the suspended contact must be mounted from at least one direction. Such sensors also typically provide a series of different outputs after each impact, because the weighted contact continues to oscillate through several complete cycles before the impact energy dissipates. During this time, the devices are incapable of sensing further impacts.
Electromagnetic impact sensors are in wide use, particularly in automotive security systems. Traditional electromagnetic impact sensors include a sensor coil fixed relative to an object whose motion is to be detected, and a permanent magnet on an elastic support. See, for example, U.S. Pat. No. 4,584,569 to Lopez et al., issued Apr. 22, 1986 and entitled "Motion Sensitive Security System." When at rest, the sensor coil intercepts a portion of the magnetic flux associated with the permanent magnet. When the object is struck, the permanent magnet will be displaced from its rest position, and the flux intercepted by the sensor coil is altered, thereby inducing an electrical potential, V, across the coil according to the equation V=N(d.PHI./d.tau.), where N is the number of turns of wire in the coil, .PHI. is the magnetic flux, and .tau. is time. When the induced potential, V, exceeds a certain threshold, an alarm or other event is triggered.
Traditional electromagnetic impact sensors have significant disadvantages, however, primarily resulting from the fact that they rely on movement of a permanent magnet to sense impact. The resonant frequency of the sensor is dependent on the mass of its magnet and the spring constant of its support. Because magnetizable materials typically are not very dense, a relatively large magnet must be used. This places a lower limit on the size of prior electromagnetic impact sensors. In addition, because of the size required, large ranges of motion are possible, and a relatively large separation between the permanent magnet and the coil is necessary. If, for size reasons, the permanent magnet is constrained from moving in any given direction, then the sensor will not be sensitive to impacts in that direction.
These shortcomings of traditional electromechanical and electromagnetic impact sensing devices highlight the need for an improved sensor which is simple, reliable, inexpensive, and small, yet sensitive to movement in all directions and tunable to various frequencies for different applications.